The testing of such a signal path generally involves the feeding of an original bit pattern of limited length, preferably produced by a pseudorandom generator, to an input end of the test object and the bit-by-bit comparison of the transmitted pattern, extracted from the output end of the test object, with a reference sequence identical with the original pattern produced by the same generator or by a duplicate thereof; see, for example, commonly owned U.S. Pat. No. 3,965,294. Whenever the bit comparator detects a disparity, it emits an error pulse which can be counted to provide a measure of the bit-transmitting quality. With a total number of N bit cycles in the test pattern, an error rate R=n/N where n is the number of error pulses.
The conventional evaluation of transmission lines and the like on the basis of this error rate R fails to take into account, however, the fact that in many instances the individual bit errors are not all independent of one another. In PCM transmission using 8-bit code combinations or bytes, for example, an initial bit error often entrains consequential errors in the same code combination. Moreover, a faulty code word can usually be detected by parity checks, with replacement of the entire word by a corrected version regardless of the number of actual bit errors. Thus, it often appears desirable to count a bunch of closely spaced bit errors as a single error for evaluation purposes.